190 HALL AND SARDESON — THE MAGNESIAN SERIES. 



experiment of Maiignac and felt confirmed thereby in a cliemical theory 

 of the origin of dolomite. Calcium carbonate with a solution of mag- 

 nesium sulphate was heated six hours to 200 degrees centigrade under a 

 pressure of 15 atmospheres. The result was dolomite with a double 

 carbonate of calcium and magnesium. Hydrochloric acid might be 

 used in the experiment instead of sulphuric acid with a similar result 

 in dolomite. The conclusion reached from the above experiment was 

 that dolomite is found in the sea under similar conditions, namely, a 

 temperature of 200 degrees, at a depth of 200 meters, at which depth the 

 necessary pressure obtains. It is confessed, however, that this theory does 

 not explain the porous condition of many dolomite beds. It is further 

 difficult to understand how the necessary acid is developed and distrib- 

 uted throughout the sea ; besides, deep-sea soundings made within the past 

 twenty years have effectually proved that all hypotheses based on the as- 

 sumption of a high temperature in the ocean abysses have no foundation. 

 Along this line of exposition Gustav Bischof 's predilections led him as 

 he investigated the carbonate rocks, and particularly the dolomites. It 

 is not necessary to summarize his chapter on " Dolomite,"* for the last 

 sentence but one tersely states his result : 



" Taking into consideration- all the facts known with regard to dolomite, so far 

 as it occurs as a rock mass, it can only be regarded as a product of the alteration of 

 limestone in the wet way, and there is no mode of alteration that is more probable 

 than the substitution of carbonate of magnesia present in water for a portion of the 

 carbonate of lime in limestone or the extraction of the greater part of carbonate of 

 lime by the water permeating the limestone." 



We shall have occasion to refer to this conclusion on another page. 

 In 1863 Dr T. Sterry Hunt expressed his opinion that — 



"The carbonate of lime which the alkaline waters generally contain, being pre- 

 cipitated with the magnesian carbonate, the combination of these two subsequently 

 gives rise to dolomite or to magnesian limestone." f 



In his later writings Hunt opposes the theory that dolomite is formed 

 by a partial replacement of lime in ordinary limestone — 



"since beds of dolomite or more or less magnesian limestone are found alternating, 

 sometimes in thin and repeated layers, with beds of non-magnesian carbonate of 

 lime."t 



In 1879 Henry Clifton Sorby,§ in his anniversary address, discussed 

 the magnesian limestone of south Yorkshire and Nottinghamshire. He 

 holds to the replacement theory, although he is commendably cautious, 

 as this sentence shows : || 



* Elements of Chemical and Physical Geology, Paul's translation, 1859, vol. iii, pp. 155-208. 

 t Geology of Canada, 1863, p. 575. 



I Mineral Physiology and Physiography, 1891, p. 171. . 

 § Quarterly Journal of the Geological Society, 1879, London, pp. 56-93. 



II Ibid., p. 85. 



